(43e) Investigating the Genesis of Catalytic Promotion for Silica-Supported Molybdenum Oxide during Propylene Metathesis

Olefin metathesis is a robust strategy for various applications ranging from fine chemical syntheses to large-scale production of petroleum commodities. Recently, we showed that co-feeding substituted olefins (2,3-dimethyl-1-butene, i4ME) during propylene self-metathesis over silica-supported W and Mo catalysts increases reaction rates by orders of magnitude. We hypothesized the co-fed promoter (i4ME) facilitates the active site formation through a 1,2-proton shift mechanism resulting in a drastic increase in the number of active metal centers. Here, we present this kinetic and spectroscopic study to investigate further this promotion effect using silica-supported Mo catalysts. Compared with WO_x/SiO₂ catalysts, we observed similar kinetic trends with the MoO_x/SiO₂ catalysts: i) the co-fed i4ME decreases the apparent activation energy of propylene self-metathesis and ii) the apparent reaction order of propylene with a positive reaction order of i4ME. Exposing the pretreated catalyst to 1 mol.% i4ME and 10 mol.% propylene separately in transmissive in-situ Fourier-transform infrared (FTIR) spectrometer, i4ME perturbs more Si-OH groups than propylene at 50 °C, suggesting that co-feeding i4ME provides more accessible protons involved in the 1,2-proton shift mechanism. We identify a structure-function relationship in which co-fed i4ME influences the reactivity of atomically dispersed molybdate more efficiently than the reactivity of molybdate clusters. With i4ME co-feeding, the Mo loading normalized turnover frequency of catalysts with low Mo loading (1.4-3.6 wt.%) is higher than high Mo loading (6.6-14.2 wt.%). Our results from in-situ ultraviolet-visible (UV-Vis) spectroscopy and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) show the co-fed i4ME further reduces surface Mo ions (V/VI) to Mo (IV), which has been proposed either as the active site or a key intermediate formed during olefin metathesis. These observations have reinforced our hypothesis that i4ME can facilitate active site formation during propylene self-metathesis and shown that only atomically dispersed molybdate on the catalyst surface is active for propylene self-metathesis.